The present invention generally relates to pull type tools, and more specifically relates to a load control mechanism for pull type tools, such as a pull type tool for removing a rivetless nut plate.
Fasteners used in association with fluid tanks, such as fuel tanks or water tanks, for aircraft or the like present a problem in preventing leakage through the openings for the fasteners. Fasteners used in such installations may include a nut plate and a nut which are part of an assembly. There are many different design configurations of nut plates being used today. Two major classes are riveted nut plates and rivetless nut plates.
In riveted nut plates, two rivets are employed for attaching the body of the nut plate to the workpiece. To eliminate the potential for leakage, sealant is used between the structure and the nut plate.
With regard to rivetless nut plates, some designs provide that a sleeve is flared against a workpiece. One example of this type of rivetless nut plate is disclosed in U.S. Pat. No. 4,732,518, which is hereby incorporated herein by reference in its entirety. The '518 patent illustrates the insertion of a sleeve inside a workpiece against heavy interference forces and then deformation of the sleeve to produce flaring of the end of the sleeve. The sleeve has a serration/lobe configuration thereon with the serration/lobe configuration being long and tapered such that the serrations/lobes extend into the walls of the workpiece. The tapered feature, length and specific geometry are necessary to make installation possible with the method of installation which was chosen for its application. The main object of the '518 patent with its tapered and extended serration/lobe configuration was to enhance the fatigue life of the workpiece by distributing the load throughout the workpiece and providing expansion due to the insertion of the sleeve into the workpiece, and to cold work the material adjacent the perimeter of the workpiece aperture.
Other nut plate designs do not rely on flaring of the sleeve. U.S. Pat. Nos. 5,096,349, 5,245,743, 5,405,228 and pending U.S. application Ser. No. 10/272,721 (filed Oct. 17, 2002) and Ser. No. 10/929,701 (filed Aug. 30, 2004) disclose rivetless nut plate designs which do not rely on flaring of the sleeve, and these five items are hereby incorporated herein by reference in their entirety. While some rivetless nut plate designs rely on adhesive for attaching the nut plate to the structure, the designs disclosed in the five items cited above rely on heavily cold-worked holes and high interference engagement utilizing a hardened pin as the installation tool to expand a sleeve element into engagement with a workpiece structure. Because of high level expansion, the friction forces created are intended to retain the nut plate and provide expected mechanical properties.
Once installed, most rivetless nut plates are removed by drilling out the flared sleeve portion, or by drilling out the rivets while holding the nut plate. Drilling out the rivets and the sleeve from the holes requires a special skill. The procedure, aside from being cumbersome and time consuming, leaves behind contaminating metal chips. Additionally, it often enlarges the hole size requiring an oversized nut plate for replacement. Safety considerations, for drilling out the old nut plate, are required to prevent damage to surrounding structure and the operator.
U.S. patent application Ser. No. 11/218,076 discloses a plurality of tools which can be used to remove rivetless nut plates. Some of the designs disclosed in U.S. patent application Ser. No. 11/218,076 provide that a pull type power tool, such as a riveter, can be used to remove a rivetless nut plate. However, the actual working load necessary to complete the removal operation is much lower than the load provided by the riveter. Unfortunately, there is typically no way to control and limit the load that is applied.
Riveters are typically used to install break-stem fasteners. In such case, the load which is ultimately applied by the riveter during operation is effectively limited by the breakage of the stem at a certain load, thereby preventing the tool from overload. However, in applications where a riveter is not being used to install a break-stem fastener, such as where the riveter is being used to remove a rivetless nut plate, there is no such inherent load control feature, and there exists a critical need to control the working load in order to provide means for safe operation and prevent tool breakage.
While riveters are available in different power ratings, the load provided by the available selection of riveters is too high to be used to remove rivetless nut plates. The high load leads to overloading of the parent material (i.e., the workpiece in which the rivetless nut plate is installed), or certain tool components. It is disadvantageous to overload the tool during operation, as overloading will cause critical failure of the tool.